Variable capacitor crash sensor

ABSTRACT

A pair of electro-mechanical sensors mounted between the vehicle bumper and frame exhibit a capacitance related to the position of the bumper relative to the frame. During a collision, the changing capacitance of the sensors is detected by control circuitry to determine the velocity of the bumper relative to the frame at impact. If the velocity at impact is greater than a predetermined value for a predictable interval of time, where the predictable interval of time varies with the velocity at impact, a signal is provided for inflating a cushion.

United States Patent Gillund et a1.

[15] 3,703,300 [451 Nov. 21, 1972 [54] VARIABLE CAPACITOR CRASH SENSOR72 Inventors: Arden G. Gillund, Oak Creek; Dan K. Holtshouse,.Milwaukee;John A. Moretti; Vincent A. Orlando, both of Greendale, all of Wis.

[73] Assignee: General Motors Corporation,-

Detroit, Mich.

[22] Filed: July 2, 1971 211 Appl. No.: 159,169

[52] U.S. Cl. ..280/150 AB, 180/91, 317/246,

73/517, 293/1 [51] Int. Cl. ...B60r 21/08 [58] Field of Search ..l80/82,91, 99, 103;

280/150 AB; 340/61, 52 H, 262; 73/518,

[56] References Cited UNITED STATES PATENTS 2,025,719 12/1935 Blau etal. ..73/517 R X 3,172,684 3/1965 Isaac ..280/150 AB 3,495,474 2/ 1970Nishimura et a1 1 80/91 X 3,560,922 2/1971 Wilson ..340/6l 3,596,5928/1971 Trissnak et a1. ..317/246 3,633,159 1/1972 Dillman et al...l80/9l Primary Examiner-Kenneth H. Betts Assistant Examiner-GeorgeSteube Attorney-Eugene W. Christen et a1.

[57] a ABSTRACT A pair of electro-mechanical sensors mounted between thevehicle bumper and frame exhibit a capacitance related to the positionof the bumper relative to the frame. During a collision, the changingcapacitance of the sensors is detected by control circuitry to determinethe velocity of the bumper relative to the frame-at impact. If thevelocity at impact is greater than a predetermined value for apredictable interval of time, where the predictable interval of timevaries with the velocity at impact, a signal is provided for inflating acushion.

7 Claims, 6 Drawing Figures PATENTEDMum m2 SHEET 3 BF 3 ATTORNEYtrically operated actuator means such as a squib which is energized froma source of electrical power under the control of one or more mechanicalimpact switches or accelerometer means. Y

It is well known to mount bumpers or other impact bars on vehicles byenergy absorbing devices which ab sorb the energy of impact forces; bythe displacement of a medium between variable volume chambers. Suchenergy absorbing devices can only absorb the energy of the impact forcesup to a predetermined absorption limit.'When this limit isreached orexceeded the device loses structural integrityand deformation of thevehicle will occur.

It is an object of the present invention to provide an electronic systemfor actuating an inflatable cushion in conventional structure and areinterconnected adjacent their free ends by front frame cross member 16.A bumper or impact bar 18 extends across the front of the vehicle and issupported thereon by a pair of like energy absorbing devices 20.

' Each of the devices includes an outer cylindrical member 22 and aninner cylindrical member 24 which are telescopically arranged. Themember 22 is attached to the frame rail 14 while the member 24 issecured to the impact bar 18. The specific details of the device 20 arenot necessary for understanding the present invention, and therefore, inthe interest of brevity will not be described further. Details of suchdevices may be obtained from copending application 'Ser; No. 91,008,Jackson et al., filed Nov. 19,1970 and'assigned to the assignee of thepresent invention.

a vehicle in the event of a collision exceeding a predetermined velocityand for initiating inflation of the cushion at a time which is prior tovehicle deformationand dependent on the impact velocity.

In accordance with the present invention an elec- I tromechanical sensoris mounted between the frame of the vehicle and the impact bar. Thecapacitance of the sensor changes as the distance between the impact barand the vehicle frame changes. The capacitance of the sensor is detectedby control circuitry which converts the capacitance to a dc signal whichis differentiated to determine the velocity of the impact bar relativeto the vehicle frame. When a predetermined velocity is exceeded for aninterval of time a control signal is developed for firing the actuatorfor initiating inflation of the cushion. The time interval decreaseswith in-' creasing impact velocity. The system also includes means forinhibiting firing of the actuator from extraneous noise pulses ortransients in the power supply.

Other objects and advantages of the present invention will be apparentfrom the following detailed description which sould be read inconjunction with the drawings in which:

FIG. 1 is a partial perspective view of a vehicle body embodying aninflatable occupant restraint system according to the present invention;

FIG. 2 is a sectional view taken along lines 2-2 of FIG. 1;

FIG. 3 is a sectional view of the variable capacitance sensor employedin the present invention;

FIG. 4 is a functional block diagram of the system;

FIG. 5 is a detailed schematic diagram of the system;

FIG. 6 shows the waveform output of the multivibrator employed in FIG.4.

Referring now to the drawings and initially to FIGS. 1 and 2, a vehiclebody generally designated 10 and being of either the body-frame type orof the integral body type includes a frame or frame extension 12 havinga pair of forward frame rails 14. Such rails are of A conventionalinflatable occupant restraint system generally designated 26 is mountedon the vehicle. The system includes a pressure vessel 28 containing airunder pressure and having an outlet sealed by a rup turable diaphragm,not shown, the rupture of which is initiated by electrically fireddetonators, not shown. The sealed pressure vessel 28 communicates with amanifold 30 and a diffuser 32 both of conventional structure. Aninflatable cushion 34 for the center and right front seat passengers ofthe vehicle 10 is inflated from the diffuser in a conventional manner.The cushion 34 is shown inflated for'purposes of clarity only. Referencemaybe had to Ser. No. 36,080, Clove et al., filed May II, 1970, now U.S.Pat. No. 3,618,978, and assigned to the assignee of the presentinvention, for the details of the manner in which the cushion isnormally stored and inflated.

Mounted between the impact bar 18 and the frame rails 14 are identicalcapacitive sensors 36 and 36'. The capacitive sensor 36 is shown in FIG.2 and includes a cylindrical plunger 38 and inner and outer cylindricalmembers 40 and 42. The plunger 38 is mounted to the impact bar 18through a spherical mounting generally designated 44, while the member42 is mounted by bracket 45 to the frame rail 14 through a sphericalmounting generally designed 46. The mountings 44 and 46 permit limitednon-axial movement of the plunger 38 relative to the members 40 and 42to prevent jamming during impact. The member 42 is closed at one end bya cover 48. The inner member 40 is located within the member 42 by asupport 50 which electrically insulates the member 40 from the member42. The plunger 38 carries a bearing 52 which slidably engages themember 40. The members 40 and 42 are provided with bearings 54 and 56respectively which re slidably engaged by the plunger 38. The sensor 36is protected from the environment by a boot 58 secured to the member 42and plunger 38 by means of clamps 60 and 62. The member 42 carries amicroswitch 64 having an armature 66 normally located in an opening 68in the plunger 38 when the plunger 38 is in its fully extended andnormal position. The armature 66 is actuated to close the microswitchupon a predetermined displacement of plunger 38 relative to member 42.The plunger 38 and the members 40 and 42 form a variable capacitor, thecapacitance of which increases as the member 40 moves within the plunger38. The plunger 38 and member 42 are electrically connected and groundedto the outer portion of a coaxial connector 72 and through the vehicleframe in order to form a high frequency shield to prevent electricalnoise from entering the capacitor. The member 40 is connected with theinner portion of the connector 72 for connection with a source ofvoltage. The plunger 38 is also electrically connected to the member 42through a conductive spring 70 which insures that the plunger 38 and themember 42 remain at the same potential. The plunger 38 and the member 40thus form a first capacitor which is electrically connected in parallelwith a second capacitor formed by the members 40 and42. Movement of themember 40 within the plunger 38 increases the capacitance of the firstcapacitor while decreasing the capacitance of the second capacitor.However, the gap between the plunger 38 and the member 40 issubstantially less than the gap between the members 40 and 42 so thatthe total effect of inward movement of the member 40 is to increase thecapacitance of the sensor 36 as measured at connector Referring now toFIG. 4, a functional block diagram of the control circuitry forinitiating air cushion inflation is shown and includes a power supply 80which supplies current to an actuator 82 under the control of a siliconcontrolled rectifier (SCR) 84. The circuitry for firing the SCR 84includes a variable duty cycle multivibrator generally designated 86which produces a substantially rectangular output wave whose ratio ofon-time'to off-time is proportional to the capacitance of whichever oneof the sensors 36, 36' exhibits the largest capacitance. The output ofthe multivibrator 86 is averaged by a filter 88 which provides a dcoutput signal proportional to the capacitance and consequentlyproportional to the position of the impact bar 18 relative to the frame14. The position information output of the filter 88 is fed to adifferentiator 90 which develops an output proportional to the velocityof the impact bar 18 relative to the frame 14. The output of thedifferentiator 90 is connected with an integrator 92. The integrator 92does not integrate until a predetermined threshold velocity has beenreached after which the integration process begins. A detector 94 sensesthe output of the integrator 92 and fires the SCR 84 provided the outputof the integrator 92 remains for a time interval dependent on the impactvelocity. An inhibit circuit 96 senses the voltage in the power supply80 and inhibits firing of the SCR during tum-on and tum-off of the powersupply when transients occur that could cause an unintentional firing ofthe SCR 84. The inhibit circuit 96 is also connected with the output ofthe filter 88 and inhibits the SCR 84 whenever an abnormally largevelocity is detected, such as might occur as a result of spurioussignals in the circuit.

Referring now to FIG. 5, a more detailed schematic diagram of the systemis shown. The power supply 80 includes the 12 volt vehicle battery 83which is connected through an ignition switch 85, a choke 87, and adiode 89 to a pair of networks 91 and 93, which maintain first andsecond regulated voltages at the junctions 95' and 97 respectively. Thenetwork 91 comprises a filter including resistors 98, 100 and capacitors102, 104. A zener diode 106 is connected in series with a resistor 108which regulates the voltage at the junction 95. The network 93 comprisesa filter including resistors 110, 112 and capacitors 114, 116. A zenerdiode 118 regulates the voltage at the junction 97.

The electrically operated actuator or squib 82 is connected across thebattery 83 through the ignition 85, the choke 87, the microswitch 64,and the anode/cathode electrodes of the SCR 84. A resistor 65 isconnected in parallel with the microswitch 64.

The variable duty cycle multivibrator 86 comprises a conventionalmonostable or single shot multivibrator 126 connected between thejunction 97 and ground. The single shot 126 is triggered to itssemistable state from a trigger circuit generally designated 128. Thesingle shot 126 remains in its semistable state for a fixed durationwhereupon it returns to its stable state. The interval of time duringwhich the single shot 126 remains in its stable state is dependent uponthe capacitance of the sensors 36, 36 and more specifically that sensorwhich exhibits the largest capacitance. The sensor 36 is connectedbetween ground and the junction 95 through a relatively large chargingresistor 130 and a relatively small resistor 132. A capacitor 134 isconnected in parallel with the capacitor 36 and resistor 132. A resistor132 and capacitor 134 provides an impedance match for the cableconnecting the sensor 36 with the control circuitry. A voltagecomparator 135 is connected between the junction 97 and ground and has areference voltage established at its inverting input by voltage dividingresistors 136 and 138 connected between the junction and ground. Thenon-inverting input to the comparator is connected to ground through theresistor 132 and the sensor 36. A transistor 140 has its emittergrounded and its collector connected to the resistor 130. The base oftransistor 140 is connected to the output of the single shot 126 througha current limiting resistor 142, a speed up capacitor 144 and a diode146 and to ground through, a resistor 148. The sensor 36' is connectedwith similarcircuitry to that previously described with regard to sensor36. Corresponding components are designed by prime numbers. The outputsof the comparators 135 and 135' provide the two inputs to an AND gate150. The output of the AND gate 150 is connected to the trigger input ofthe single shot 126. The output of the AND gate 150 is also connectedthrough a diode 151 and a resistor 152 to the base of transistors 140and 140 to insure starting of the multivibrator 86.

When the voltages at the inverting input of the comparators 135 and 135are exceeded by the voltage developed across the sensors 36 and 36respectively, the AND gate 150 is enabled and triggers the single shot126 which provides an output of fixed duration as shown in FIG. 6.During this time the output of the single shot 126 renders thetransistors 140 and 140 conductive to discharge the sensors 36 and 36'respectively. When the single shot 126 times out and reverts to itsstable state, the sensors 36 and 36' begin to charge again through theresistors 130, 132 and 130, 132' respectively. Under normalnon-collision conditions the single shot 126 produces a substantiallyrectangular wave output having a fixed on-time of l microsecond and anoff-time of approximately 0.5 microseconds. Since the single shot 126cannot be triggered until both comparators 135 and 135' have switched,the time interval during which the single shot 126 is in its stable oroff state during a collision is dependent on the sensors 36 or 36'exhibiting the largest capacitance and therefore, experiencing thegreatest relative velocity between the impact bar 18 and the frame 14.The variable off-time of the single shot 126 is not dependent on powersupply voltage variations because both of the RC networks as well as thevoltage reference networks for the comparators 135 and 135 are connectedto the junction 95. If the junction 95 decreases for instance, thereference level decreases but the voltage to which the RC networkcharges also decreases causing the offtime of the single shot 126 toremain unchanged for a given capacitance.

The output of the multivibrator 86 is inverted by an inverter bufferstage generally designated 154 comprising a transistor 156. Thetransistor 156 has its emitter grounded, its collector connected to thejunction 97 through a resistor 158 and its base connected to the outputterminal of the single shot 126 through a current limiting resistor 160and a speed-up capacitor 162.

The filter 88 comprises a resistor 164 and a capacitor 166 and isconnected across the collector/emitter electrodes of the transistor 156and converts the rectangular wave output from the buffer stage 154 to anaverage dc level. The filter 88 is connected to an emitter followerbuffer stage comprising a transistor .168 and a resistor 170.

The differentiator network 90 comprises capacitor 172 and a variableresistor 174 connected across the emitter follower resistor 170. Thecurrent through the exceeding the threshold velocity before switching ofthe comparator 94 can occur is thus dependent on both the value of theresistor 190 and the magnitude of the impact velocity. For a particularvalue of resistance of resistor 190 the time interval decreases withincreasing impact velocity. The output of the comparator 94 is fedthrough a current limiting resistor 192 and a plurality of voltagedropping diodes 194 to the gate electrode of the SCR 84. When thecomparator 94 switches the SCR 84is fired. A noise filter comprising aresistor 196 and a capacitor 198 is connected across the gate and anodeelectrodes of the SCR 84.

The inhibit circuitry 96 comprises a transistor 200 having its collectorconnected to the junction 95 through a resistor 206, its emittergrounded and its base connected to the junction between the zener diode106 and the resistor 108. The collector of the transistor 200 isconnected to the base of a transistor 208 through a diode 210. Thetransistor 208 has its emitter grounded and its collector connected tothe gate electrode of the SCR 84. When power is initially applied to thesystem by closure of the ignition switch 85 the transistor 208 isrendered conductive from the junction 95 through the resistor 206 anddiode 210 thus grounding the gate electrode of the SCR 84. As the powersupply 80 capacitor 172 is the differential of the input voltage and 1is proportional to impact velocity or relative velocity between theimpact bar 18 and the vehicle frame 14. The integrator 92 comprising anoperational amplifier 178 having its inverting input terminal connectedto the junction between the capacitor 172 and the resistor 174 and itsnon-inverting input terminal connected to the junction 97. A diode 180and an integrating capacitor 182 connect the output of the comparator178 to the inverting input terminal. The current from the output of theamplifier 178 through the diode 180 and resistor 174 maintains thevoltage at the inverting input terminal equal to the voltage at thenon-inverting input terminal. When current flow through thedifferentiating capacitor 172 is sufficient to maintain the voltage atthe inverting put terminal of the amplifier 178 equal to i thatestablished at the non-inverting input of the amplifier 178, the diode180 is back biased and current flow commences through the integratingcapacitor 182. The resistor 174 establishes the velocity threshold whichmust be exceeded before the integration process begins. No current flowsthrough the capacitor 182 until current flow through the capacitor 172is indicative of a velocity greater than that set by the variableresistor 174. When the integration process begins the voltage output ofthe amplifier 178 begins to decrease. The rate of decrease of the outputvoltage is dependent on the current flow through the capacitor 182 andconsequently the magnitude of the impact velocity.

The output of the amplifier 178 is connected to ground through a diode184 and a resistor 186. The voltage developed across the resistor 186 isapplied to the inverting input terminal of the detector or voltagecomparator 94. The non-inverting input to the comparator 94 is connectedto a variable resistor 190 which establishes a reference level belowwhich the output of the amplifier 178 must drop in order to cause thecomparator 94 to switch. The time interval which must elapse subsequentto the detection of an input velocity reaches its operating voltage thezener diode 106 will conduct sufiicient base drive current through thetransistor 204 to render this transistor conductive which robs basecurrent from the transistor 208 rendering it nonconduc'tive andthereafter permitting the SCR to be fired. By inhibiting the SCR 84during turn-on of the system, transients which build up in the powersupply are prevented from inadvertently firing the SCR 84. The junctionbetween the zener diode 106 and resistor 108 is also 7 connected to asecond pair of transistors 212 and 214. The transistor 214 is renderedconductive through the resistor 216 when the system is initially turnedon to energize an indicator lamp 218 through the resistor 65. When thepower supply is stabilized the transistor 212 is rendered conductive toturn off the transistor 214 and extinguish the indicator lamp 218.

The transistor 208 is also connected with circuitry generally designed220 which renders the transistor 208 conductive thereby inhibitingoperation of the SCR 84 whenever a velocity input is detected which isgreater than'that which is reasonably expected such as might occur as aresult of momentary open circuit in the connections to the 36, 36'. Thecircuitry 220 includes a differentiator network comprising a capacitor222 and a resistor 224 connected across the emitter follower resistor170. An integrator 226 comprising an amplifier 228 has its invertinginput connected to the differentiating capacitor 222 and itsnon-inverting input connected to the junction 97. The output of theamplifier 228 is connected through a resistor 230 in parallel with anintegrating capacitor 232 to the inverting input of the amplifier 228.The output of the amplifier 228 is fed to the inverting input of avoltage comparator 234 having its non-inverting input connected to ajunction between voltage dividing resistors 236, 238. The output of thevoltage comparator 234 is connected through a diode 240 to the base oftransistor 208 and through a resistor 242 to ground. The integrator 226and voltage comparator 234 operate similarly to the integrator 92 andvoltage comparator 94 previously described. The resistor 224 establishesa threshold velocity input to the amplifier 228 at some predeterminedvalue, for example, 75 mph. Any input to the amplifier 228 representinga velocity above 75 mph will cause the voltage comparator 224 to switchand render the transistor 208 conductive to inhibit the SCR 84,

The overall operation of the system is as follows: When the ignitionswitch 85 is closed the transistors 208 and 214 are rendered conductiveto inhibit the SCR 84 and energize the lamp 218. When the power supply80 has stabilized, the lamp 218 is extinguished and the inhibit isremoved from the gate of the SCR 84. The single shot 126 produces asubstantially rectangular wave output due to the alternate charging anddischarging of the sensors 36 and 36. Under normal conditions thecapacitance of the sensors 36 and 36 is constant at a fixed value whichestablishes an off-time for the single shot 126 of approximatelyone-half the on-time. The output of the single shot 126 is inverted bythe buffer stage 154 and filtered by the filter 88 to established anaveraged dc input voltage to the differentiator 90. The current throughthe differentiating capacitor 172 and resistor 174 is insufficient toback bias the diode 180 in the integrator 92 so that no current flowsthrough the integrating capacitor 172. Consequently, the voltage at theinverting input of the voltage comparator 94 is greater than thereference voltage established at the non-inverting input of the voltagecomparator 94 and no trigger voltage is applied to the gate of the SCR84. During a collision, the microswitch closes and the capacitance ofboth sensors 36 and 36' increases, increasing the time necessary tocharge the sensors 36 and 36' to a voltage sufficient to switch thecomparators 135 and 135 which has the effect of lengthening the off-timeof the single shot 126. Thus the ratio of on-time to off-time of thesingle shot 126 is decreased. Since the triggering of he single shot.126 is dependent on sensors 36 and 36' being charged above the referencevoltage established at the comparators 135 and 135 respectively, thetriggering of the single shot 126 is under the control of the sensor 36,36' exhibiting the largest rate of change of capacitance. As thecapacitance of the probe 36, 36' increases so does the dc level at theinput to the differentiator 90. If the impact velocity as represented bythe changing dc level is greater than the threshold velocity establishedby the resistor 174 in the differentiator network 90 the diode 184 isback biased and current begins to flow through the integrating capacitor182. If the velocity of impact is below the threshold established at theresistor 174 the diode 180 remains forward biased and no current flowsthrough the integrating capacitor 182. The time required for the voltageacross the resistor 186 to drop below the reference voltage establishedby the resistor 190 is dependent on the velocity of impact. When thevoltage across the resistor 186 drops below the voltage established bythe resistor 190 the voltage comparator 94 switches triggering the SCR84 and tiring the squib 82. Thus the squib 82 is fired only if themicroswitch 64 has closed and the threshold velocity is exceeded for acertain interval of time, which interval of time is dependent on theimpact velocity. The movement of the impact bar 18 relative to the frame16 which is required in order to close the microswitch 64 preventsactuation of the squib 82 from a short duration high velocity impactwhich causes little or no relative movement between the impact bar 18and the frame 16. In addition to the above enumerated requirements forfiring the squibs 82 the circuitry 220 prevents firing of the SCR 84 inthe event of the detection of an unreasonably high input velocityresulting from. malfunctioning of the sensors 36, 36' or other spurioussignals introduced in the circuitry.

Having thus described my invention what we claim 1. In a passengerconveyance the combination comprising an inflatable cushion,electrically operated actuator means for inflating said cushion;

variable capacitance impact sensor means mounted on said vehicle andcomprising members which are relatively displaceable upon impact of saidvehicle to vary the capacitance of the sensor means in proportion to therelative displacement of said members; I

means for converting the capacitance of said sensor means to a signalproportional to the relative velocity of said members;

means responsive to said signal for operating said actuator means aninterval of time after the relative velocity of said members exceeds apredetermined velocity.

2. In an automotive vehicle the combination comprising, a source ofpressure fluid, an inflatable cushion, electrically operated actuatormeans for communicating the cushion with said source, a bumper impactbar receptive to impact forces, energy absorbing means supporting theimpact bar on the vehicle frame;

variable capacitance impact sensor means mounted between said impact barand said vehicle frame, said sensor means exhibiting a change incapacitance in response to a change in the relative distance between theimpact bar and the vehicle frame;

means responsive to the capacitance of said sensor means for developinga signal proportional to the relative velocity of said impact bar andsaid vehicle frame; means responsive to said signal for operating saidactuator means an interval of time after the said relative velocityexceeds a predetermined velocity.

3. The combination defined in claim 2 wherein said sensor meanscomprises an outer cylindrical member mounted to said frame and havingan axis, an inner cylindrical member supported within said outer memberin axial alignment therewith and electrically insulated therefrom, meansfor connecting said inner member to a source of potential, a cylindricalplunger mounted at one end to said impact bar, bearing means mountingthe other end of said plunger for axial movement between said inner andouter members, said inner member and said plunger forming a firstcapacitor, said inner and outer members forming a second capacitorelectrically in parallel with said first capacitor the total capacitanceof which increases as the distance between the impact bar and thevehicle frame decreases.

4. The combination defined in claim 3 further comprising a normally openswitch supported by said outer member and including an armature normallybiased to the open position and movable to the closed position inresponse to relative movement of said plunger and said outer member,means connecting said switch in series with said actuator means toinhibit operation of said actuator means until said switch is closed.

5. In an automotive vehicle the combination comprising, a source ofpressure fluid, an inflatable cushion, electrically operated actuatormeans for communicating the cushion with said source, a bumper impactbar receptive to impact forces, energy absorbing means supporting theimpact bar on the vehicle frame;

' variable capacitance impact sensor means mounted between said impactbar and said vehicle frame, said sensor means exhibiting a change incapacitance in response to a change in the relative distance between theimpact bar and the vehicle frame;

multivibrator means including said sensor means and producingasubstantially rectangular output wave whose ratio of on-time tooff-time is proportional to the capacitance of said sensor means;

means for averaging the output of said multivibrator means to produce adc signal proportional to the capacitance of the sensor means;

differentiator means responsive to said dc signal to provide a signalproportional to the velocity of impact;

means responsive to the output of said difierentiator means foroperating said actuator means an interval of time after the velocity ofimpact exceeds a predetermined velocity.

6. In an automotive vehicle the combination comprising, a source ofpressure fluid, an inflatable cushion, electrically operable actuatormeans for communicating the cushion with said source, a bumper impactbar. receptive to impact forces, energy absorbing means supporting theimpact bar on the vehicle frame,

a plurality of electro-mechanicalsensor means mounted between saidimpact bar and said vehicle, each sensor means forming a capacitor andeach capacitor exhibiting a change of capacitance as the relativedistance between the impact bar and the vehicle frame changes;

variable duty cycle multivibrator means including each of saidcapacitors and producing a substantially rectangular wave whose rationof on-time to off-time is proportional to the capacitance of thecapacitor exhibiting the largest capacitance;

means for averaging the output of said multivibrator means to produce adc signal proportional to the position of said impact bar relative tosaid vehicle frame;

differentiator means for differentiating said dc signal to provide asignal proportional to the velocity of said impact bar relative to saidvehicle frame;

integrator means connected to said differentiator means and operative todevelop a control voltage when the velocity of said impact bar relativeto said vehicle frame exceeds a predetermined velocity, said controlvoltage attaining a predetermined magnitude in a time interval dependenton the impact velocity;

voltage detector means for operating said actuator means in response tosaid control voltage attaining 'd d te n dm ni de. 7. i' he b ca m in ati o defin ed i n claim 6 wherein said variable duty cycle multivibratormeans comprises a source of voltage, resistance means connected inseries with each of said sensor means respectively across said source,voltage comparator means producing respective outputs when respectiveones of said sensor means are charged above a reference voltage, ANDfunction performing logic means responsive to the outputs of each ofsaid comparators for providing a trigger pulse when all of said sensormeans have charged above the respective voltage references, a monostablemultivibrator triggerableto its semistable state by the output from saidgate means, means responsive to the output of said monostablemultivibrator for discharging said sensor means during the semistablestate of said multivibrator whereby the time duration of the stablestate of said multivibrator is determined by the capacitance of thelarger of said plurality of sensor means.

' s a a a s

1. In a passenger conveyance the combination comprising an inflatablecushion, electrically operated actuator means for inflating saidcushion; variable capacitance impact sensor means mounted on saidvehicle and comprising members which are relatively displaceable uponimpact of said vehicle to vary the capacitance of the sensor means inproportion to the relative displacement of said members; means forconverting the capacitance of said sensor means to a signal proportionalto the relative velocity of said members; means responsive to saidsignal for operating said actuator means an interval of time after therelative velocity of said members exceeds a predetermined velocity. 1.In a passenger conveyance the combination comprising an inflatablecushion, electrically operated actuator means for inflating saidcushion; variable capacitance impact sensor means mounted on saidvehicle and comprising members which are relatively displaceable uponimpact of said vehicle to vary the capacitance of the sensor means inproportion to the relative displacement of said members; means forconverting the capacitance of said sensor means to a signal proportionalto the relative velocity of said members; means responsive to saidsignal for operating said actuator means an interval of time after therelative velocity of said members exceeds a predetermined velocity. 2.In an automotive vehicle the combination comprising, a source ofpressure fluid, an inflatable cushion, electrically operated actuatormeans for communicating the cushion with said source, a bumper impactbar receptive to impact forces, energy absorbing means supporting theimpact bar on the vehicle frame; variable capacitance impact sensormeans mounted between said impact bar and said vehicle frame, saidsensor means exhibiting a change in capacitance in response to a changein the relative distance between the impact bar and the vehicle frame;means responsive to the capacitance of said sensor means for developinga signal proportional to the relative velocity of said impact bar andsaid vehicle frame; means responsive to said signal for operating saidactuator means an interval of time after the said relative velocityexceeds a predetermined velocity.
 3. The combination defined in claim 2wherein said sensor means comprises an outer cylindrical member mountedto said frAme and having an axis, an inner cylindrical member supportedwithin said outer member in axial alignment therewith and electricallyinsulated therefrom, means for connecting said inner member to a sourceof potential, a cylindrical plunger mounted at one end to said impactbar, bearing means mounting the other end of said plunger for axialmovement between said inner and outer members, said inner member andsaid plunger forming a first capacitor, said inner and outer membersforming a second capacitor electrically in parallel with said firstcapacitor the total capacitance of which increases as the distancebetween the impact bar and the vehicle frame decreases.
 4. Thecombination defined in claim 3 further comprising a normally open switchsupported by said outer member and including an armature normally biasedto the open position and movable to the closed position in response torelative movement of said plunger and said outer member, meansconnecting said switch in series with said actuator means to inhibitoperation of said actuator means until said switch is closed.
 5. In anautomotive vehicle the combination comprising, a source of pressurefluid, an inflatable cushion, electrically operated actuator means forcommunicating the cushion with said source, a bumper impact barreceptive to impact forces, energy absorbing means supporting the impactbar on the vehicle frame; variable capacitance impact sensor meansmounted between said impact bar and said vehicle frame, said sensormeans exhibiting a change in capacitance in response to a change in therelative distance between the impact bar and the vehicle frame;multivibrator means including said sensor means and producing asubstantially rectangular output wave whose ratio of on-time to off-timeis proportional to the capacitance of said sensor means; means foraveraging the output of said multivibrator means to produce a dc signalproportional to the capacitance of the sensor means; differentiatormeans responsive to said dc signal to provide a signal proportional tothe velocity of impact; means responsive to the output of saiddifferentiator means for operating said actuator means an interval oftime after the velocity of impact exceeds a predetermined velocity. 6.In an automotive vehicle the combination comprising, a source ofpressure fluid, an inflatable cushion, electrically operable actuatormeans for communicating the cushion with said source, a bumper impactbar receptive to impact forces, energy absorbing means supporting theimpact bar on the vehicle frame, a plurality of electro-mechanicalsensor means mounted between said impact bar and said vehicle, eachsensor means forming a capacitor and each capacitor exhibiting a changeof capacitance as the relative distance between the impact bar and thevehicle frame changes; variable duty cycle multivibrator means includingeach of said capacitors and producing a substantially rectangular wavewhose ration of on-time to off-time is proportional to the capacitanceof the capacitor exhibiting the largest capacitance; means for averagingthe output of said multivibrator means to produce a dc signalproportional to the position of said impact bar relative to said vehicleframe; differentiator means for differentiating said dc signal toprovide a signal proportional to the velocity of said impact barrelative to said vehicle frame; integrator means connected to saiddifferentiator means and operative to develop a control voltage when thevelocity of said impact bar relative to said vehicle frame exceeds apredetermined velocity, said control voltage attaining a predeterminedmagnitude in a time interval dependent on the impact velocity; voltagedetector means for operating said actuator means in response to saidcontrol voltage attaining said predetermined magnitude.